Analysis of purified phagolysosomal membrane and plasma membrane from Acanthamoeba castellanii shows chemical differences in the phospholipid and lipophosphonoglycan content between the two membranes that are consistent with known functional differences. The cytological mechanism for an alteration in chemical composition and its relation to membrane recirculation are being studied. Stereological analysis of electron micrographs of endocytosing amebas were performed to determine quantitative relationships between phagolysosomal membrane, plasma membrane and a related cytoplasmic membrane component, small vesicles. The results indicate that small vesicles could be the vehicle for membrane recirculation and also provide the most likely mechanism for the observed alteration of the chemical composition of phagolysosome membrane. Future studies will continue to explore the problem of membrane composition and recirculation by attempting to follow specific chemical components from the plasma membrane through the phagolysosome.